The invention relates generally to semiconductor devices, and more particularly to semiconductor devices having piezoelectric or the like pressure sensing elements.
It can be appreciated that some semiconductor devices comprise elements or features that actually deflect or otherwise move or deform in the operation of the devices. For example, some types of sensors, such as pressure sensors and/or accelerometers/decelerometers, for example, include a portion of a semiconductor substrate (e.g., silicon) that moves in response to changes in pressure and/or acceleration or deceleration, for example. Such devices can be used in automotive applications, for example, to determine the severity of a side impact collision as well as whether an individual is present in a passenger seat of the automobile, and thus whether both a driver side and a passenger side airbag should be deployed, for example. Similarly, such sensors could be used to determine the pressure within different systems of an automobile, such as within the engine, for example, and thus whether a ‘check engine’ light should be activated or illuminated.
Such devices generally comprise one or more piezoelectric elements whose electrical characteristics change as a function of an encountered force, (e.g., amount of deflection). The devices may employ piezoresistance, for example, such that a resistance changes as a function of deflection (e.g., due to a change in pressure). Such piezoresistive elements can be formed, for example, through diffusion or doping processes whereby the resistance of these treated areas changes as a function of applied stress. It is not uncommon for such devices to be arranged in a Wheatstone bridge configuration to provide an enhanced output signal. In any event, sensing regions generally require associated circuitry, such as support circuitry for calibration and/or compensation, among other things, for example. Signals produced by the piezoresistive elements can, for example, be sensed by conductive traces and then processed by associated circuitry and forwarded via leads to external circuitry, which can, for example, use the sensed signals to determine whether to deploy an airbag.
It can also be appreciated that there is an ongoing desire to decrease the size of semiconductor devices while concurrently reducing the cost of such devices, such as through streamlining an associated fabrication process, for example. Nevertheless, sensing regions are conventionally formed in silicon by selectively etching the back side of certain areas of a substrate so that the thickness of the substrate in these areas is sufficiently reduced so that the silicon deflects or deforms in response to an applied stress. This selective etching process, however, requires multiple steps that protract the fabrication process. Moreover, the small selectively thinned areas prohibit associated circuitry from being formed close to the sensing regions, where forming associated circuitry close to sensing regions would be desirable because it fosters enhanced performance and allows sensing devices to be more compact.
By way of further example discrete piezoresistive sensing resistors (e.g., metal foil, silicon resistors in a Wheatstone bridge configuration) that are bonded to an engineered mechanical structure, and that have separate signal conditioning electronics mounted in a stress free zone (e.g., ASIC mounted on a separate PCB) can be used for force sensors where the strain gages are mounted on a load cell/beam, or for a pressure sensor where the strain gages are mounted on a metal diaphragm. This configuration has certain disadvantages in terms of packaging size, packaging complexity, and/or cost due to the use of separate strain gage resistors and signal conditioning ASIC. This arrangement does have an advantage, however, of the piezoresistive sensing resistors not coming into contact with the pressure media being sensed; that is, the sensing resistors are sensing the stress transmitted through a diaphragm that is in direct contact with the pressure media.
By way of further example, in piezoresistive sensing resistors that are integrated with signal conditioning electronics on a selectively thinned single silicon substrate, the applied strain can be isolated from the signal conditioning circuitry. In this arrangement the thinned silicon section forms a diaphragm that is in direct contact with the pressure media being sensed. While the integrated strain gage and signal conditioning structure provides desirable packaging size and cost advantages, this type of integrated structure may have applications limited to pressure sensing due to the fact that the design must allow the pressure media to be in direct contact with the silicon.